CO₂ hydrogenation to methanol catalyzed by Ni₅Ga₃ metal alloy

CO2 can be converted to a great number of value-added chemicals such as methanol, dimethyl ether, formic acid and long chain hydrocarbons. Methanol is one of the most essential compounds because it does not only serve as fuel for internal combustion engines, but also is an important starting material for light olefins production^[1]. CO2 molecules are ubiquitous however extremely stable, thus proper catalysts are sought to accelerate methanol synthesis process. Methanol is industrially produced in the presence of Cu-ZnO-Al2O3 catalysts which were first discovered in the 1920s. The competitive price and reasonable CO2 conversion of these catalytic materials make them outstand among many other catalysts. However, the drawbacks towards the commercial ones are also obvious such as limited methanol selectivity which is contributed to the by-product CO generated via reverse water gas shift reaction. High pressure and temperature are also required to active CO2 molecules when conventional copper catalysts are employed. In order to enhance the catalytic performance, metal oxide promotors such as ZrO2^[2], CeO2^[3], TiO2 and Ga2O3^[4] have been introduced into copper based catalysts. Novel Nickel-Gallium bimetallic catalyst for CO2 hydrogenation to methanol has been reported recently^[5]. Even though the calculation process was simplified by reducing number of energy parameters in methanol synthesis, the results still indicated that Ni-Ga alloy presented comparable good catalytic performances, compared with Cu-ZnOAl2O3 catalysts in terms of CO2 conversion and methanol selectivity under atmospheric pressure. The most inspiring advantage of novel Ni5Ga3 is to provide a “low pressure-low temperature” reaction path. In this research, we aimed to synthesize Ni5Ga3 through a novel pathway, where catalysts could be prepared conveniently and productively. This was expected a promising strategy for industrial production of the novel catalyst. The XRD pattern of as-obtained Ni5Ga3 metal alloy was illustrated in Fig. 1(a). The reflections exhibited only typical Ni5Ga3 crystal plane (112), (040), (402), (422), (422) and (223), respectively, and no other obvious phase such as Ni3Ga or NiGa. The sharp peaks also showed the fact that considerable pure Ni5Ga3 samples can be obtained. From the SEM image of Ni5Ga3 shown in Fig. 1 (b), the particles tended to agglomerate. The average particle size observed from SEM was approximately200 nm. From EDS measurement, it was found that nickel element took 63.46 % and gallium 36.54 % of mole percentages, which was roughly in accordance to the stoichiometric number ratio between nickel and gallium in Ni5Ga3. Thus, both XRD and SEM showed that pure Ni5Ga3 has been achieved with the novel synthesizing method. The catalytic performance for CO2 hydrogenation to methanol was tested in a fixed bed reactor. In the entire experiment, the feed gas was H2/CO2 mixed gas in the ratio of 3:1. Exhausted gas and generated liquid were collected and analyzed by gas chromatography (7890B, Agilent technologies). CO2 conversion and methanol selectivity can be calculated accordingly, (1) (2) where Fin is the total inlet flow of the mixed H2/CO2 gas, fCO2, fCO and fCH4 present the molar fraction of CO2, CO and CH4 in the exhaust gas. In the emission gas, CO, CH4 and CH3OH were the only products observed in the GC analysis, thus the carbon-based calculation is only based on CO2 and three kinds of products. CO2 conversion and methanol selectivity in CO2 hydrogenation to methanol at 30 bar were investigated and summarized in Fig. 2. The results showed the methanol selectivity decreased as temperature increased, but the CO2 conversion exhibited a bell shape at 200 ^oC, followed by an increase at 300 ^oC. The fact that byproducts CO and CH4 favored at high temperatures resulted in the continuous reduction of methanol selectivity and the increase of CO2 conversion at high temperatures^[6],[7]. Compared with a commercial catalyst, the optimized temperature for methanol catalyzed by Ni5Ga3 decreased from 250 ^oC to 200 ^oC, and the selectivity was observed incredibly high within the entire temperature range. Hereby, Ni-Ga metal alloy in Ni5Ga3 form was prepared via a novel method in this work. This catalyst exhibited attractive catalysis properties at the low reaction temperature, i.e., significant methanol selectivity and favorable methanol yield.